As a fixing apparatus used in an electrophotographic image forming apparatus such as a copying machine, a printer, and the like, a heat roller type fixing apparatus is frequently used. The heat roller type fixing apparatus includes a pair of rollers (a fixing roller and a pressing roller) which are pressed against each other, wherein heating means constituted of halogen heaters disposed in both the rollers or a halogen heater disposed in one of the rollers heats the pair of rollers at a predetermined temperature (fixing temperature), and a recording paper on which an unfixed toner image is formed is fed to a pressing section (fixing nip section) of the pair of rollers, and the recording paper is allowed to pass through the pressing section so as to fix the toner image by heat and pressure.
In a fixing apparatus provided in a color image forming apparatus, it is general to use an elastic roller having an elastic layer which is made of silicon rubber or the like and which is provided on a fixing roller surface layer. The elastic roller is used as the fixing roller, so that the fixing roller surface is elastically deformed corresponding to an uneven surface of the unfixed toner image and is in contact with the toner image so as to cover the toner image. This allows the color unfixed toner image whose toner amount is larger than that of monochrome to be favorably heated and fixed. Further, due to strain release of the elastic layer which occurs in the fixing nip section, it is possible to improve a releasing property with respect to color toner which is more likely to offset than monochrome toner. Further, a nip shape of the fixing nip section has a concave upward (on the side of the fixing roller) (that is, the nip shape is a so-called inverse nip shape), so that it is possible to more favorably strip paper. As a result, it is possible to strip paper without using any stripping means such as a stripping protrusion (self stripping), so that it is possible to prevent insufficient image formation which is caused by the stripping means.
However, in the fixing roller having the elastic layer, the elastic layer cannot sufficiently conducts heat. Thus, in case where the heating means is provided in the fixing roller, heat is less efficiently conducted, so that it takes longer time to warm up. In case where the process is carried out at higher speed, the fixing roller cannot follow the process.
As a method for solving these problems, a technique in which external heating means is brought into contact with the fixing roller surface so that the fixing roller is heated from the outside (external heat fixing process) is known. For example, each of below-described Patent Documents 1 and 2 proposes an external belt heat fixing process using an endless belt as external heating means.
Note that, in a conventional external belt heat fixing type fixing apparatus, a plurality of rollers (belt suspending rollers) each of which suspends an endless belt are used as tension rollers each of which exerts a tension to the endless belt or it may be so arranged that a tension roller for the endless belt is provided to exert a tension to the endless belt (see Patent Documents 1 and 2).
(Patent Document 1)
Japanese Unexamined Patent Publication No. 198659/2004 (Tokukai 2004-198659) (Publication date: Jul. 15, 2004)
(Patent Document 2)
Japanese Unexamined Patent Publication No. 189427/2005 (Tokukai 2005-189427) (Publication date: Jul. 14, 2005)
However, in case where one of the belt suspending rollers is used as a tension roller, this requires a complicate mechanism for exerting a tension to the endless belt. Further, it is impossible to keep the plural belt suspending rollers in parallel to each other, so that the endless belt has a greater deviation force (force which causes the endless belt to move in a direction perpendicular to a rotational direction). As a result, it is difficult to control snaking of the belt.
Further, in case where a tension roller is additionally provided on the outside of the endless belt, the number of parts required therein becomes increase, so that the arrangement is complicated. Further, there is such a problem that: the tension roller becomes a thermal load, so that the thermal efficiency drops.